Based on the two-timescale channel property, the large timescale channel \(\bf {G}\)  and the small timescale channels \({\bf {h}}_d\)  and \({\bf {h}}_r\)  can be respectively estimated in different timescales, which can indeed significantly reduce the average pilot overhead. However, the channel estimation for \(\bf {G}\)  is still challenging. In [1]}, the BS should work in the full-duplex mode, where different antennas are required to transmit and receive pilots simultaneously to estimate \(\bf {G}\) . In [2]}, the complexity for user scheduling and the overhead for the \(U_1\) -RIS-\(U_2\)  cascaded channel feedback are not negligible.
